1. Field of the Invention
The present invention relates to a mobile communication system, a channel synchronization establishing method, and a mobile station.
2. Related Background Art
Recently, the mobile communication systems utilizing the code division systems such as the W-CDMA (Wideband-Code Division Multiple Access) system are in practical use, in order to increase the utilization efficiency of frequency bands for radio communications. In such mobile communication systems, a common frequency band is used in a plurality of cells and it is thus difficult to identify each cell where mobile equipment is located, by referring to the frequency band. In order to identify each cell, therefore, it is common practice to multiply downward channels (hereinafter referred to as “downlink channels”) by scrambling codes (spreading codes) different among the cells.
In such mobile communication systems, the cell search operation described below has to be performed in order for a mobile station to communicate with a desired base station. Specifically, the mobile station first detects a radio frame boundary (hereinafter referred to simply as a “frame boundary”) of the downlink channel of the base station to be connected. Then the mobile station detects the scrambling code used by the base station.
On the other hand, in the case of the orthogonal frequency and code division multiplex (OFCDM: Orthogonal Frequency and Code Division Multiplex) system being one of typical multiplex systems using a plurality of carriers for downlink channels, the cell search is performed by three-step processing, in order to implement a fast cell search. The three-step processing includes symbol boundary detection, frame boundary detection, and scrambling code detection. The frame boundary detection process of the second step also involves a simultaneously executed operation of separating the scrambling codes into several groups (hereinafter referred to as “scrambling code groups”) and detecting a scrambling code group. This operation reduces the number of scrambling codes as candidates in the third step and thereby increases the speed of the third step (the detection process of scrambling codes).
FIG. 1 is a diagram showing a conventional frame architecture to which the OFCDM system is applied. In the conventional OFCDM base cell search, as shown in FIG. 1, processing described below was carried out in order to perform the detection of scrambling code group simultaneously with the frame boundary detection in the second step. Specifically, in order to permit the detection of frame boundary, the base station sent additional SCHs (Synchronization Channels) 101, 102, 103 transmitted in the form of a special symbol sequence known to the mobile station, without multiplying the downlink channels by the scrambling codes, and the mobile station calculated the cross correlation between a received signal and the known SCH symbol sequence. (CN-1 . . . C1 identifying segments of the frame.) In order to permit the simultaneous detection of scrambling code group with the detection of frame boundary, a plurality of SCH symbol sequences were prepared and scrambling code groups were made correspondent to the respective SCH symbol sequences.